1. Field of the Invention
This invention relates to the preheating of refractory-lined ladles for containing and transporting molten metal and, more particularly, to a system and method for monitoring the heat content of a ladle during preheating and indicating accurately when the ladle refractories are uniformly heated throughout, and particularly to such a system and method in which it is determined when the ladle is so heated by measuring the slope of the heat input rate (or the fuel flow rate) over time and, especially, the second derivative of a variation-corrected rate of change of heat input rate to the ladle.
2. Description of the Prior Art
In a steelmaking shop, brick or cast refractory-lined ladles are used to transport liquid steel from a steelmaking furnace to a treatment section of the shop or to a forming operation such as continuous casting. In the latter case, it is necessary that the casting operation be carried out continuously, so several ladles may rotate through the shop simultaneously. The thermal state of the ladle has a direct and significant impact on heating of the ladle and also on liquid steel temperature loss during transport of the ladle from the steelmaking furnace to secondary steelmaking processes and to a continuous caster.
Such ladles may heat up when filled with liquid metal because of the heat absorbed from the melt by the ladle refractory lining. On the other hand, the ladles cool off when empty. The length of time during which a ladle is empty is highly variable and unpredictable. For example, delays due to a major ladle repair taking many hours to complete may result in a very cool ladle which, if used in that condition, will cause relatively high loss of the liquid metal temperature. In continuous casting operations, liquid steel, as introduced into the caster tundish, may be only about 40.degree. F. above the metal liquidus temperature. In such case, one cannot afford to lose significant and unanticipated heat to the ladle.
On the other hand, over-heating of a ladle is inefficient and costly and may result in increased refractory damage.
Accordingly, ladle preheating is an important common practice in the metals manufacturing field, and serves to normalize heat losses for ladles taken out of the rotational use cycle for repair and for ladles first introduced into the use cycle, and to minimize thermal stresses in the ladle refractory due to pouring hot liquid metal into a cool refractory lining.
Usually a gas-fired burner is used to inject a flame into the interior of the ladle, for example when the ladle is positioned on its side on a horizontal preheating stand. Gas-fired ladle preheaters are represented, for example, by U.S. Pat. Nos. 4,359,209; 4,229,211; 4,014,532, and 3,907,260. Heating a ladle with electrical power also is known, for example as shown in U.S. Pat. No. 4,394,566.
FIG. 1 of the present application illustrates a typical prior art method of changing fuel gas flow to a ladle preheater in respect to control temperature (actual ladle refractory hot face temperature as measured by a thermocouple in the ladle) and set point temperature (predetermined desired ladle hot face temperature). As indicated by FIG. 1, it is usual to use a maximum fuel flow rate during an initial preheating time period when the ladle is relatively cool, then gradually to decrease fuel flow rate after the set point temperature is reached and until the ladle is fully heated. A typical time for control temperature to reach the set point temperature is about 2 hours, and a typical time to reach a fully heated condition of the ladle refractory is about 20 hours, as also indicated in FIG. 1.
Currently, control of a ladle preheater usually is based on feedback from a thermocouple located in the preheater lid. This thermocouple measures the average hot face temperature of the ladle refractory. Initially, when the ladle first is placed on the preheater, the burner fires at maximum capacity to input heat as rapidly as possible. As the hot face temperature approaches the set point temperature, the burner is throttled back so that the set point temperature is maintained and not overshot. That is, as the ladle hot face approaches the set point temperature, the fuel flow rate is reduced so that the rate of heat input matches the rate at which heat is being absorbed into the refractory, as shown in FIG. 1.
Practically, fuel flow rate can be considered to be equivalent to the heat input rate to a ladle during preheating. The principal difference is that some heat from the burning fuel, e.g. natural gas, is lost, primarily to off-gases (flue gases). Thus, heat input rate is a somewhat more accurate measure of ladle heat content than is gas flow rate.
Exemplary of such prior art, U.S. Pat. No. 1,512,008 discloses methods and apparatus for maintaining working temperature, in, e.g. an electrically-heated furnace, by varying the rate of heat input rapidly in response to wide variations in thermocouple-determined furnace temperature, for example, by quickly raising the temperature near a desired level, then varying the heat input rate slowly as the temperature nears the desired value.
U.S. Pat. No. 4,223,873 discloses a direct flame ladle preheating system including a control circuit to maintain combustion gases at a predetermined temperature and to adjust fuel-air ratio in order to maximize combustion and minimize oxygen remaining in the combustion gases.
U.S. Pat. No. 4,718,643 relates to ladle preheating in which flow of fuel and oxygen is controlled responsive to ladle temperature to increase heat input during an initial preheating phase and to insure maximum system efficiency during a soaking phase.
U.S. Pat. No. 4,462,698 relates to ladle preheating in which a radiation pyrometer is used to measure the (hot face) ladle refractory for control of gas flow rate.
Such prior art methods are appropriate for controlling the surface temperature of the ladle refractory during preheating, but they do not indicate when a preheated ladle has absorbed enough heat so that the temperature losses of the liquid metal will be consistent and controllable. Thus these earlier practices fall short of indicating ladle readiness for use after preheating because the temperature distribution within the ladle lining thickness is unsteady due to a cyclic heat input (e.g. when liquid steel is poured into the ladle) and cooling periods (e.g. when the ladle is empty). For example, when a ladle is full of liquid steel, the refractory is exposed to a heat source of high temperature, e.g. about 2800-3000.degree. F. in contact with and moving against the inside surface of the refractory lining of the ladle. After casting or pouring the liquid steel out of the ladle, the empty ladle is exposed to the atmosphere for a significant period of time during which the inside surface of the refractory lining cools, typically to about 1400.degree. F. or less. Further unpredictable variables, such as ambient temperature and wind conditions in the steelmaking shop, significantly affect ladle refractory and shell temperatures. These thermal variables are not taken into account by such prior art practices. The same is true of changes in refractory thickness over the course of several use cycles, due to erosion of the refractory, which causes a loss in insulating capacity and hence a change in heat capacity of the ladle and the rate of heat input during preheating.
Measuring the temperature of the steel shell of the ladle also does not provide an effective way of measuring or controlling the rate of heat input to the ladle. For example, a ladle, recycled, say 11/2 hours after casting its contents, may be put on a preheater because it is considered to be too cold. The inside surface of the refractory lining may be about 1200.degree. F. and the working lining (the lining next to a bath of liquid metal and underlain with a thinner safety lining) may have lost a significant amount of heat, but the shell temperature may be about 650.degree. F.--which would indicate that the ladle is ready for service--but in fact the ladle is cold and, if used in this condition, will cause significant heat loss from the liquid metal. Thus, similarly to ladle hot face temperature, ladle shell temperature will not reliably indicate overall thermal conditions of the ladle refractories.
A practical monitoring and signalling system is needed for more accurately indicating to an operator when a preheated ladle is ready for service, i.e. when the ladle is heat soaked throughout the refractory lining and thus is hot enough to guarantee minimum and consistent heat loss from the molten metal.